The technical portion of the work of an optician consists in mounting a pair of correcting ophthalmic lenses in an eyeglass frame selected by a wearer. Such mounting comprises three main operations:                acquiring the shape of a rim or of a half-rim of the eyeglass frame;        centering the lens, which consists in positioning and orienting the lens appropriately in front of the corresponding eye of the wearer; and then        machining the lens, which consists in cutting or shaping its outline to the desired shape, given the shape of the rim or half-rim and given defined centering parameters.        
The specific object of the optician is to shape the ophthalmic lens so that it fits mechanically and pleasingly in the shape of the rim or half-rim of the selected frame, while also ensuring that the lens performs as well as possible the optical function for which it is designed.
With rimmed eyeglass frames, shaping includes a beveling operation that consists in forming an engagement ridge along the edge face of the lens, which ridge is for engaging in a groove that runs around the inside face of the rim of the eyeglass frame.
With half-rimmed eyeglass frames, the shaping comprises a grooving operation that consists in forming an engagement groove along the edge face of the lens, part of the groove serving to be engaged on a ridge that runs along the inside face of the half-rim of the eyeglass frame. The lens is held in the half-rim by means of a string that is connected to the ends of the half-rim and that is received in the remaining portion of the engagement groove.
For reasons of appearance, it is generally desirable to place the ophthalmic lens in the rim or the half-rim of the eyeglass frame in such a manner that the front face of the lens is as close as possible to being flush with the front margin of the rim or the half-rim.
On these lines, it is known for example from document WO 2007/128902 to begin by feeling the bottom of the bezel of a rim of a rimmed eyeglass frame longitudinally in order to acquire the shape of the outline of the rim, and then to feel a section of the bezel transversely in order to acquire the distance between the bottom of the bezel and the front margin of the rim. During the beveling operation, the engagement ridge is then made so as to lie at a distance from the front face of the lens that is constant and equal to the acquired distance. Thus, once the ophthalmic lens is engaged in the rim, its front face is substantially flush with the front margin of the rim.
Nevertheless, that method presents accuracy that is not very satisfactory. A rim of an eyeglass frame generally presents a section that is of non-uniform shape (for example the rim may be slightly twisted), such that the distance between the bottom of the bezel and the front margin of the rim varies. Thus, when said distance is measured at only one cross-section of the rim, a lens that is obtained after machining that presents a front face that is generally flush with the front margin of the rim only in that section that was felt.
That same document also describes feeling a plurality of cross-sections of the rim of the eyeglass frame, thereby making it possible to acquire the distance between the bottom of the bezel and the front margin of the rim in a plurality of zones around the rim. Nevertheless, that method is awkward and time-consuming to implement.